solve for a matrix



Hello,

I want to solve a linear algebraic system like this:
y1 - y2 = 0
y3 + 2*y4 = 0
y4 = 1

which has the solution y1=y2, y3=-2, y4=1 

but I have stored it in matrix form A*x=b, with the matrices (not in maxima format):
A=(1,-1,0,0; 0,0,1,2; 0,0,0,1), x=(y2,y2,y3,y4) and b=(0,0,0,1)

Solve does not work with matrices, so is there: 
1. a way of converting the matrix system Ax=b into a list 
or 
2. a way of getting the solution Ax=b using the matrix form?

Regards,
Nijso